i) Field of Invention
This invention relates to a method of moisture cross-linking of vinyl chloride homopolymers and copolymers. The process comprises a grafting of PVC resin with vinyltrialkoxysilane in the presence of an initiator, preferably a peroxide, followed by the hydrolysis of the alkoxy group of the silane to yield a cross-linked structure through the formations of Si--O--Si bonds.
ii) Description of Prior Art
The cross-linking of a polymer improves the physical properties of the product at elevated temperature. This includes the improvement in the thermal dimensional stability, resistance to thermal deformation, resistance to solvents, and stress cracking, etc. The usual method to cross-link poly(vinyl chloride), i.e. PVC, is to expose the PVC construct containing cross-linking agents to a high energy radiation source such as an electron beam (Salmon, J. Appl. Polym. Sci., 16, 671 (1972); Valdiserri and Reed, Rubber World, 170 (5), 40 (1974)). This method is suitable only for thin layers, since for thicker material the surface regions are degraded before significant cross-linking occurs in the interior. Because such radiation is inherently non-discriminatory, in addition to the cross-linking it tends to initiate degradation reactions in PVC that are exacerbated by any subsequent heat treatments, either in processing or in application. The conventional chemical method which is used for cross-linking polyethylene, that is with organic peroxides, is not satisfactory since the degradation of PVC predominates over cross-linking (Miller, Ind. Eng. Chem., 51, 1271 (1959); Mori and Nakaamura, J. Polym. Sci., A-1, 9, 639 (1971); Mori and Nakamura, J. Polym. Sci. Polym. Chem. Ed., 16, 1981 (1978); Mora and Nakamura, J. Appl. Polym. Sci., 22, 2685 (1978); Hjertberg, Dahl, and Sorvik, J. Appl. Polym. Sci., 37, 1239 (1989)).
Another method of cross-linking polymers is by modifying the polymer chain by chemically grafting active groups such as organofunctional silanes (Cartasegna, Rubber Chem. Technol., 59, 724 (1985)), which in the presence of water form a cross-linked product. The silane cross-linking of polyethylene is well known as Sioplas-E (Scott and Humpries, Mod. Plast., 50, 82 (1973)) and Monosil (Maillefer, Technical Paper, Reg. Tech. Conf. Soc. Plast. Eng., p. 12 (1979)) systems which are commercially established processes. The process involves the grafting of a silane group on to the backbone of polyethylene by using vinylalkoxysilanes in the presence of a peroxide initiator, by reactive compounding or, by copolymerization of ethylene with silane in the reactor. (Neste Polyetylen AB, Sweden, Information Booklet, VISICO, IN 0180 1987 03/E; Bullen, Capaccio, Frye, and Brock, Br. Polym. J., 21, 117 (1989)). This reaction is initiated by free radicals produced through the thermal decomposition of an organic peroxide. This is summarized in the reaction sequence shown below: ##STR1##
After fabricating the end product, the cross-linking is achieved through the action of water when a Si--O--Si bridge is formed by the combination of two or more pendent silane groups, as shown below. In order to accelerate the cross-linking reaction a condensation catalyst is also employed. ##STR2##
Several attempts have been made to cross-link PVC by the silane method. In 1969, the Dow Corning Co. patented a process of copolymerizing vinyl chloride with vinyl silane (Lewis, Collins, and Malani Br. Pat. 139, 248 (1969)). A second patent on this subject was obtained by Dow Corning for a process using amino silanes to cross-link the PVC (Saam and Thomas, Br. Pat. 1,485,263 (1973)). In the latter work aminosilane was grafted to the PVC by reactive processing; several other references using amino silane as a cross-linking agent are in the literature but the process has not been exploited commercially to any extent (Shindo and Hirai, Makromol. Chem., 155, (1972); Kelnar, PhD Thesis, Institute of Chemical Technology, Prague, (1988); Hearn, Baird, Nethsinghe and Gilbert, Polym. Commun., 31 (5),194 (1990); Kellner and Schatz, Sb. Vysk. Sk. Chem.-Technol. Praze, Polym., Chem.,Vlastnosti Zprac., S17, 31 (1988)). More recently, the use of mercaptosilanes as a grafting agent has also been reported (Kellner and Schatz, Sb. Vysk. Sk. Chem.-Technol. Praze, Polym., Chem., Vlastnosti Zprac., S17, 31 (1988); Fujikura Cable Works Ltd., Japan Kokai 565,854 (1981), Chem. Abstr., 94, 209757 (1981); Fujikura Cable Works Ltd., Japan Kokai 5,796,048 (1982), Chem. Abstr., 97, 183070 (1982); Fujikura Cable Works Ltd., Japan Kokai 5,796,049 (1982), Chem. Abstr., 97, 183071 (1982); Fujikura Cable Works Ltd., Japan Kokai 5,787,438 (1982), Chem. Abstr., 97, 199068 (1982); Yamamoto, Ogawara and Hirokawa, Japan Kokai 63010605 (1988), Chem. Abstr., 109(6), 38487w; Schatz, Kelner and Vysoky Ger. Offen. 3719151 (1987), Chem. Abstr., 108(24), 205319f). Kelner and Schatz have successfully cross-linked PVC by using mercapto-alkoxy silanes with minimum degradation of PVC (Kelnar and Schatz J. Appl. Polym. Sci., 48, 657, 669 (1993)). Blends of PVC with other grafted polymers have also been used as a means to achieve the properties of the cross-linked PVC (Japan Kokai Tokyo Koho JP 60 42429 (85 42429) 1985; Hasue and Yamane, Japan Kokai 61, 243,848 (1986), Chem. Abstr.,107, 8477 (1987); Hayashi Japan. Kokai JP 53021249 (1978), Chem. Abstr., 89(10), 75893q).
In spite of considerable attempts during the last 25 years, the silane grafting process for cross-linking PVC has not been commercially exploited to any extent. Presumably this is due to the degradation of PVC which accompanies the grafting process, making the end product unacceptable. The present invention describes the moisture cross-linking of a thermoplastic PVC compound using vinyltrialkoxysilane with a minimum amount of degradation making the end product and this process commercially viable.